(a) Field of the Invention
The present invention relates to an ink jet recording head having a piezoelectric substrate and, more particularly, to an ink jet recording head of piezoelectric type suitable for use in a printer, facsimile, copying machine etc. The present invention also relates to a method for manufacturing such an ink jet recording head.
(b) Description of the Related Art
Ink jet recording heads are classified in two categories based on the principle of the ink ejection. The first category is called a thermal ink jet type or a bubble jet type described in Patent Publication No. JP-B-61(1986) -59913, for example. The described ink jet recording head comprises a thermal head on which a plurality of thermal elements is arranged, and a pressure chamber having an ink nozzle disposed to each of the thermal elements for ejecting liquid ink. In operation, the thermal elements are energized to heat the liquid ink thereon for generating bubbles, the pressure of which ejects the liquid ink from the ink nozzles.
The first type has the advantage in that a thermal head can be fabricated having a large number of nozzles arranged in a high density by using a photolithographic technique. However, it has also the disadvantage in that some ingredients in the liquid ink heated up to above 300.degree. C. for generation of bubbles are likely to be deposited on the thermal elements after some continuous ejection period to cause a malfunction. Moreover, the thermal stress or cavitation generated by the heated ink may cause damages in the thermal elements or cause a pinhole in the protective film for the thermal elements, which reduces the lifetime of the ink jet recording head.
The second category is called a piezoelectric type described in Patent Publication No. JP-B-53(1978)-12138, for example. This type of ink jet recording head comprises a pressure chamber formed by a piezoelectric element which receives liquid ink therein and is communicated to ink nozzles and an ink supply tube. The piezoelectric element is energized during operation for controlling the volume of the pressure chamber to eject the liquid ink from the nozzles.
The second type has the advantages in that a variety of liquid inks can be used in the recording head and has a long lifetime. However, it has the disadvantage in that it is difficult to arrange a large number of piezoelectric elements in a high density to achieve a high density recording.
Patent Publication No. JP-A-6(1994)-143564 proposes a high density ink jet recording head of the piezoelectric type. Referring to FIG. 1, the proposed head comprises a piezoelectric planar substrate 40, a top plate 44, and a plurality of ink channels 41bc, 41de, . . . and a plurality of dummy channels 42ab, 42cd, . . . which are alternately arranged on the main surface of the planar substrate 40 and covered by the top plate 44. Before operation, liquid ink is filled only in the ink channels 41bc, 41de, . . . . In addition, the walls 43b, 43c, 43d, 43e, . . . of the piezoelectric substrate 40 separating the channels are polarized beforehand by using electrodes 48bc, 48cd, 48de, . . . formed on the surfaces of the respective channels, in the direction of arrows 47, each of which is directed from a dummy channel to an adjacent ink channel.
In operation, a driving pulse is applied to a specified channel (or to the electrode of a specified channel, more accurately), while the dummy channels are maintained at a ground potential, to expand the side walls of the specified channel, which changes the volume of the specified channel for ejection of the liquid ink therefrom as an ink droplet.
The proposed ink jet recording head mentioned above has a problem in generation of cross-talk, wherein the speed and the size of the ink droplet differ depending on the number of ink channels concurrently driven by a driving pulse.